Method of depositing thin films using microwave energy

ABSTRACT

An improved method of depositing thin films onto a substrate with microwave energy by operating at substantially the minimum of the pressure-power curve for the particular geometry of reaction vessel and composition of reaction gases being utilized.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 854,247filed on Apr. 21, 1986, now U.S. Pat. No. 4,664,937, which is acontinuation of U.S. application Ser. No. 725,616 filed Apr. 22, 1985,now U.S. Pat. No. 4,615,905, which is a continuation of U.S. applicationSer. No. 423,424 filed Sept. 24, 1982, now U.S. Pat. No. 4,517,223issued May 14, 1985.

BACKGROUND OF THE INVENTION

This invention relates to a method of making amorphous semiconductoralloys having improved photoresponsive characteristics and devices madetherefrom. The invention more particularly relates to a method of makingsuch alloys and devices by plasma deposition from reaction gases whereinthe, plasmas are excited by microwave energy. The invention has its mostimportant application in making improved photoresponsive alloys anddevices for various photoresponsive applications includingphotoreceptive devices such as solar cells of a p-i-n, p-n, Schottky orMIS (metal-insulator-semiconductor) type; photoconducting medium such asutilized in xerography; photodetecting devices and photodiodes includinglarge area photodiode arrays.

Silicon is the basis of the huge crystalline semiconductor industry andis the material which has produced expensive high efficiency (18percent) crystalline solar cells for space applications. Whencrystalline semiconductor technology reached a commercial state, itbecame the foundation of the present huge semiconductor devicemanufacturing industry. This was due to the ability of the scientist togrow substantially defect-free germanium and particularly siliconcrystals, and then turn them into extrinsic materials with p-type andn-type conductivity regions therein. This was accomplished by diffusinginto such crystalline material parts per million of donor (n) oracceptor (p) dopant material introduced as substitutional impuritiesinto the substantially pure crystalline materials, to increase theirelectrical conductivity and to control their being either of a p or nconduction type. The fabrication processes for making p-n junctioncrystals involve extremely complex, time consuming, and expensiveprocedures. Thus, these crystalline materials useful in solar cells andcurrent control devices are produced under very carefully controlledconditions by growing individual single silicon or germanium crystals,and when p-n junctions are required, by doping such single crystals withextermely small and critical amounts of dopants.

These crystal growing processes produce such relatively small crystalsthat solar cells require the assembly of many single crystals toencompass the desired area of only a single solar cell panel. The amountof energy necessary to make a solar cell in this process, the limitationcaused by the size limitations of the silicon crystal, and the necessityto cut up and assemble such a crystalline material have all resulted inan impossible economic barrier to the large scale use of the crystallinesemiconductor solar cells for energy conversion. Further, crystallinesilicon has an indirect optical edge which results in poor lightabsorption in the material. Because of the poor light absorption,crystalline solar cells have to be at least 50 microns thick to absorbthe incident sunlight. Even if the single crystal material is replacedby polycrystalline silicon with cheaper production processes, theindirect optical edge is still maintained; hence the material thicknessis not reduced. The polycrystalline material also involves the additionof grain boundaries and other problem defects.

In summary, crystal silicon devices have fixed parameters which are notvariable as desired, require large amounts of material, are onlyproducible in relatively small areas and are expensive and timeconsuming to produce. Devices based upon amorphous silicon can eliminatethese crystal silicon disadvantages. Amorphous silicon has an opticalabsorption edge having properties similar to a direct gap semiconductorand only a material thickness of one micron or less is necessary toabsorb the same amount of sunlight as the 50 micron thick crystallinesilicon. Further, amorphous silicon can be made faster, easier in largerareas than can crystalline silicon.

Accordingly, a considerable effort has been made to develop processesfor readily depositing amorphous semiconductor alloys or films, each ofwhich can encompass relatively large areas, if desired, limited only bythe size of the deposition equipment, and which could be readily dopedto form p-type and n-type materials where p-n junction devices are to bemade therefrom equivalent to those produced by their crystallinecounterparts. For many years such work was substantially unproductive.Amorphous silicon or germanium (Group IV) films are normally four-foldccordinated and were found to have microvoids and dangling bonds andother defects which produce a high density of localized states in theenergy gap thereof. The presence of a high density of localized statesin the energy gap of amorphous silicon semiconductor films results in alow degree of photoconductivity and short carrier lifetime, making suchfilms unsuitable for photoresponsive applications. Additionally, suchfilms cannot be successfully doped or otherwise modified to shift theFermi level close to the conduction or valence bands, making themunsuitable for making p-n junctions for solar cell and current controldevice applications.

In an attempt to minimize the aforementioned problems involved withamorphous silicon and germanium, W. E. Spear and P. G. Le Comber ofCarnegie Laboratory of Physics, University of Dundee, in Dundee,Scotland, did some work on "Substitutional Doping of Amorphous Silicon",as reported in a paper published in Solid State Communications, Vol. 17,pp. 1193-1196, 1975, toward the end of reducing the localized states inthe energy gap in amorphous silicon or germanium to make the sameapproximate more closely intrinsic crystalline silicon or germanium andor substitutionally doping the amorphous materials with suitable classicdopants, as in doping crystalline materials, to make them extrinsic andor p or n conduction types.

The reduction of the localized states was accomplished by glow dischargedeposition of amorphous silicon films wherein a gas of silane (SiH₄) waspassed through a reaction tube where the gas was decomposed by an r.f.glow discharge and deposited on a substrate at a substrate temperatureof about 500°-600° K. (227°-327° C.). The material so deposited on thesubstrate was an intrinsic amorphous material consisting of silicon andhydrogen. To produce a doped amorphous material a gas of phosphine (PH₃)for n-type conduction or a gas of diborane (B₂ H₆) for p-type conductionwere premixed with the silane gas and passed through the glow dischargereaction tube under the same operating conditions. The gaseousconcentration of the dopants used was between about 5×10⁻⁶ and 10⁻²parts per volume. The material so deposited including supposedlysubstitutional phosphorus or boron dopant and was shown to be extrinsicand of n or p conduction type.

While it was not known by these researchers, it is now known by the workof others that the hydrogen in the silane combines at an optimumtemperature with many of the dangling bonds of the silicon during theglow discharge deposition, to substantially reduce the density of thelocalized states in the energy gap toward the end of making theelectronic properties of the amorphous material approximate more nearlythose of the corresponding crystalline material.

The incorporation of hydrogen in the above method not only haslimitations based upon the fixed ratio of hydrogen to silicon in silane,but, more importantly, various Si:H bonding configurations introduce newantibonding states which can have deleterious consequences in thesematerials. Therefore, there are basic limitations in reducing thedensity of localized states in these materials which are particularlyharmful in terms of effective p as well as n doping. The resultingdensity of states of the silane deposited materials leads to a narrowdepletion width, which in turn limits the efficiencies of solar cellsand other devices whose operation depends on the drift of free carriers.The method of making these materials by the use of only silicon andhydrogen also results in a high density of surface states which affectsall the above parameters.

After the development of the glow discharge deposition of silicon fromsilane gas was carried out, work was done on the sputter depositing ofamorphous silicon films in the atmosphere of a mixture of argon(required by the sputtering deposition process) and molecular hydrogen,to determine the results of such molecular hydrogen on thecharacteristics of the deposited amorphous silicon film. This researchindicated that the hydrogen acted as an altering agent which bonded insuch a way as to reduce the localized states in the energy gap. However,the degree to which the localized states in the energy gap were reducedin the sputter deposition process was much less than that achieved bythe silane deposition process described above. The above described p andn dopant gases also were introduced in the sputtering process to producep and n doped materials. These materials had a lower doping efficiencythan the materials produced in the glow discharge process. Neitherprocess produced efficient p-doped materials with sufficiently highacceptor concentrations for producing commercial p-n or p-i-n junctiondevices. The n-doping efficiency was below desirable acceptablecommercial levels and the p-doping was particularly undesirable since itreduced the width of the band gap and increased the number of localizedstates in the band gap.

The prior deposition of amorphous silicon, which has been altered byhydrogen from the silane gas in an attempt to make it more closelyresemble crystalline silicon and which has been doped in a manner likethat of doping crystalline silicon, has characteristics which in allimportant respects are inferior to those of doped crystalline silicon.Thus, inadequate doping efficiencies and conductivity were achievedespecially in the p-type material, and the photovoltaic qualities ofthese silicon alloy films left much to be desired.

Greatly improved amorphous silicon alloys having significantly reducedconcentrations of localized states in the energy gaps thereof and highquality electronic properties have been prepared by glow discharge asfully described in U.S. Pat. No. 4,226,898, Amorphous SemiconductorsEquivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and ArunMadan which issued Oct. 7, 1980, and by vapor deposition as fullydescribed in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky andMasatsugu Izu, which issued on Aug. 12, 1980, under the same title. Asdisclosed in these patents, which are incorporated herein by reference,fluorine is introduced into the amorphous silicon semiconductor tosubstantially reduce the density of localized states therein. Activatedfluorine especially readily diffuses into and bonds to the amorphoussilicon in the amorphous body to substantially decrease the density oflocalized defect states therein, because the small size of the fluorineatoms enables them to be readily introduced into the amorphous body. Thefluorine bonds to the dangling bonds of the silicon and forms what isbelieved to be a partially ionic stable bond with flexible bondingangles, which results in a more stable and more efficient compensationor alteration than is formed by hydrogen and other compensating oraltering agents. Fluorine is considered to be a more efficientcompensating or altering element than hydrogen when employed alone orwith hydrogen because of its exceedingly small size, high reactivity,specificity in chemical bonding, and highest electronegativity. Hence,fluorine is qualitatively different from other halogens and so isconsidered a super-halogen.

As an example, compensation may be achieved with fluorine alone or incombination with hydrogen with the addition of these element(s) in verysmall quantities (e.g., fractions of one atomic percent). However, theamounts of fluorine and hydrogen most desirably used are much greaterthan such small percentages so as to form a silicon-hydrogen-fluorinealloy. Such alloying amounts of fluorine and hydrogen may, for example,be in the range of 1 to 5 percent or greater. It is believed that thenew alloy so formed has a low density or defect states in the energy gapthan that achieved by the mere neutralization of dangling bonds andsimilar defect states. Such larger amount of fluorine, in particular, isbelieved to participate substantially in a new structural configurationof an amorphous silicon-containing material and facilitates the additionof other alloying materials, such as germanium. Fluorine, in addition toits other characteristics mentioned herein, is believed to be anorganizer of local structure in the silicon-containing alloy throughinductive and ionic effects. It is believed that fluorine alsoinfluences the bonding of hydrogen by acting in a beneficial way todecrease the density of defect states which hydrogen contributes whileacting as a density of states reducing element. The ionic role thatfluorine plays in such an alloy is believed to be an important factor interms of the nearest neighbor relationships.

Amorphous semiconductor alloys made by the processes hereinabovedescribed have demonstrated photoresponsive characteristic ideallysuited for photovoltaic applications These prior art processes howeverhave suffered from relatively slow deposition rates and low utilizationof the reaction gas feed stock which are important considerations fromthe standpoint of making photovoltaic devices from these materials on acommercial basis. In addition, these processes result in high electrontemperature plasmas which produce, during deposition, high densities ofions. The production of these ions results in ion bombardment of thematerials or they are being deposited which can result in materialdamage.

Applicants herein have discovered a new and improved process for makingamorphous semiconductor alloys and devices. The inventive process hereinprovides substantially increased deposition rates and reaction gas feedstock utilization. Further, the process of the present invention resultsin depositions from plasmas with lower electron temperatures andsubstantially reduced ion densities and hence, substantially reduced ionbombardment and damage of the deposited materials. Still further, theprocess of the present invention results in the formation of reactivespecies not previously obtainable in sufficiently large concentrationswith prior art processes. As a result, new amorphous semiconductoralloys can be produced having substantially different materialproperties than previously obtainable. All of the above results, byvirtue of the present invention, in amorphous semiconductor alloys anddevices made therefrom having improved photoresponsive characteristicswhile being made at substantially increased rates.

As disclosed in the aforementioned referenced U.S. Pat. No. 4,217,374,new and improved amorphous semiconductor alloys and devices can be madewhich are stable and composed of chemical configurations which aredetermined by basic bonding considerations. One of these considerationsis that the material is as tetrahedrally bonded as possible to permitminimal distortion of the material without long range order. Fluorine,for example, when incorporated into these alloy materials, performs thefunction of promoting tetrahedral bonding configurations. Amorphoussemiconductor materials having such tetrahedral structure exhibit lowdensities of dangling bonds making the materials suitable forphotovoltaic applications.

Hydrogen, while smaller than fluorine, is by far less reactive.Hydrogen, as a result, in addition to promoting tetrahedral bonding,also promotes other various possible bonding configurations which canintroduce defects into the material. For example, H₂ Si bonds arepossible. These bonds are weak bonds which can thermally be brokenleaving behind dangling bonds. Also, hydrogen requires rather precisesubstrate temperature control during deposition to promote the preferredtetrahedral bonding. Therefore, hydrogen in small amounts, inconjunction with fluorine in small amounts should make the optimalamorphous semiconductor alloy. It is not hydrogen as a molecule orfluorine as a molecule, however, which is able to perform thesefunctions. It is atomic hydrogen and atomic fluorine which does. From achemical point of view in the plasma these species exist as free atomsor free radicals.

In accordance with one preferred embodiment, atomic fluorine and/orhydrogen are generated separately as free radicals and reacted withsemiconductor free radicals generated within a plasma sustained bymicrowave energy. As a result, all of the advantages of separate freeradical formation are obtained along with all of the advantages ofmicrowave plasma deposition.

In making a commercial photovoltaic device, it is necessary to provideenvironmental encapsulation of the devices to prevent undesireablechemical reactions within the device materials due to exposure tochemical species contained in the environment. For example, oxidation ofcontact materials must be prevented. Customarily, relatively heavy andthick materials such as glass or various organic polymer or plasticmaterials have been proposed to provide such protection. In accordancewith a further embodiment of the present invention, such protection isprovided which not only provides the required encapsulation, butadditionally light in weight and can be easily incorporated in a mannercompatible with the formation of the photovoltaic materials of thedevices.

SUMMARY OF THE INVENTION

The present invention provides a process for making amorphoussemiconductor alloy films and devices at substantially higher rates thanpossible in the prior art. Even though deposition occurs at the higherrates, the alloys exhibit high quality electronic properties suitablefor many, applications including photovoltaic applications.

In accordance with the invention, the process includes the steps ofproviding a source of microwave energy, coupling the microwave energyinto a substantially enclosed reaction vessel containing the substrateonto which the amorphous semiconductor film is to be deposited, andintroducing into the vessel reaction gases including at least onesemiconductor containing compound. The microwave energy and the reactiongases form a glow discharge plasma within the vessel to deposit anamorphous semiconductor film from the reaction gases onto the substrate.The reactions gases can include silane (SiH₄), silicon tetrafluoride(SiF₄), silane and silicon tetrafluoride, silane and germane (GeH₄), orsilicon tetrafluoride and germane. The reaction gases can also includegermane or germanium tetrafluoride (GeF₄). To all of the foregoing,hydrogen (H₂) can also be added. Dopants, either p-type or n-type canalso be added to the reaction gases to form p-type or n-type alloyfilms, respectively. Also, band gap increasing elements such as carbonor nitrogen can be added in the form of, for example, methane or ammoniagas to widen the band gap of the alloys.

Independent control over all of the deposition parameters can beobtained by separately generating the free radical species prior tocombination in the microwave plasma. For example, atomic fluorine and/orhydrogen can be separately generated and fed into the plasma wherein thesemiconductor free radicals are generated. The foregoing thereafterreact in the plasma and are deposited onto the substrate to form new andimproved amorphous semiconductor alloys. The semiconductor free radicalscan be generated from any of the semiconductor containing compoundspreviously mentioned.

Also, encapsulation of the photovoltaic devices is obtained bydeposition, a relatively thin layer of transparent insulating materialsover the devices. For example, the transparent materials can comprisesilicon nitride (Si₃ N₄) or silicon dioxide (SiO₂) formed for example bythe microwave glow discharge of silane and ammonia or nitrogen andsilane and oxygen respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, partly broken away, of a microwave plasmadeposition system for depositing amorphous semiconductor alloy films inaccordance with the process of the invention;

FIG. 2 is a fragmentary sectional view of an embodiment of a Schottkybarrier solar cell to illustrate one application of the amorphoussemiconductor photoreceptive alloys made by the process of theinvention.

FIG. 3 is a fragmentary sectional view of a p-n junction solar celldevice which includes a doped amorphous semiconductor alloy made by theprocess of the invention;

FIG. 4 is a fragmentary sectional view of a photodetection device whichincludes an amorphous semiconductor alloy made by the process of theinvention;

FIG. 5 is a fragmentary sectional view of a xerographic drum includingan amorphous semiconductor alloy made by the process of the invention;

FIG. 6 is a fragmentary sectional view of a p-i-n junction solar celldevice;

FIG. 7 is a fragmentary sectional view of a n-i-p junction solar celldevice;

FIG. 8 is a partial top plan view of an alternative gas feed arrangementfor the apparatus of FIG. 1 in accordance with a further embodiment ofthe present invention;

FIG. 9 is a partial top plan view of a free radical distribution systemfor the apparatus of FIG. 1 in accordance with another embodiment of thepresent invention; and

FIG. 10 is a partial perspective view of another microwave plasmadeposition system in accordance with a still further embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now more particularly to FIG. 1, a microwave depositionapparatus suitable for practicing the process of the present invention,is generally designated 10. The apparatus 10 comprises a transparenttubular chamber or vessel 12 containing a substrate 14 upon which theamorphous semiconductor alloy films are to be deposited. The substrateis heated by a radiant heater 16 and the exterior of the chamber isirradiated by a microwave energy source 17. Reaction gases passing frominlets 46 to an outlet 20 at opposite ends of the chamber 12 receivemicrowave energy from the source 17 in the area of the substrate. Thecombination of the reaction gases and the microwave energy from source17 causes the formation of a plasma in the area of the substrate,resulting in the deposition of a film 22. In accordance with the presentinvention, the reaction gases include at least one semiconductorcontaining compound to form the plasma. The temperature of the substratecan be between room temperature and about 400 degrees Centigrade and thefrequency of the microwave energy can be 2.45 Gigahertz and above andpreferably 2.45 Gigahertz. As mentioned, the combined microwave energyand the reaction gases form the plasma to permit the deposition processto proceed. During the process, the film 22 is spared from the damagingeffects of ion bombardment because of the relatively low ionconcentration in the plasma and reduced substrate self bias.

Referring now to FIG. 1 in greater detail, the tubular chamber 12comprises a central quartz portion 24 and end portions 26 at oppositeends thereof. The end portions 26 are closed by a pair of end fittings30 and 32 to complete the chamber. Each of the end fittings includes asleeve portion 34 extending from a closed end 36 to an open end portion.The open end portion is threaded to receive a collar 40 having aninwardly extending annular flange 42 at one end thereof. An o-ring (notshown) is confined in a space between the flange 42 and the end portionfor compression thereof against the quartz portion 26. An air-tight sealis provided in this way between the end fittings 30 and 32 and thetubular chamber 12.

The end fittings 30 and 32 are preferably made of stainless steel orother suitable noncorrosive metal, with the closed ends 36 being weldedor otherwise permanently joined to the sleeve portions 34. The closedend 36 of the end fitting 32 is provided with gas inlets 46 throughwhich the deposition gases are introduced into the vessel 12. An inertgas such as argon can be introduced through one inlet 46 to assist insustaining the plasma

The gas inlets 46 are preferably connected to a conventional gas rack(not shown) for establishing regulated flows of reaction gases therein.The outlet 20 is provided at the closed end 36 to the end fitting 30 forconnection to selectable first and second pumps. The first pump providesfor initial evacuation of the chamber. The second pump provideswithdrawal of unused reaction gases during operation and maintenance ofthe proper deposition pressure of 0.1 Torr or greater.

The microwave energy source 17 preferably comprises a microwave energygenerator 18 coupled to an antenna 19. The antenna 19 is housed within areflective housing 21 for concentration of the microwave energy into thechamber 12. The antenna as illustrated is a vertical antenna beingpreferably one-quarter wavelength long. The tip of the antenna justtouches the outer surface of the vessel 12 to maximize transmission ofits output to the reaction gases.

The radiant heater 16 preferably comprises a conventional resistiveheater. Heat is transmitted to the substrate 14 and the chamber 12 byradiation, without significant direct heating of the reaction gases.Alternatively, a resistive heating arrangement (not shown) may beprovided within the chamber 12 for heating the substrate 14. In thatcase, power lines for the heating element would be passed through theclosed end 36 of one of the end fittings.

In operation, the system 10 is first pumped down to below a desireddeposition pressure, such as 10⁻⁵ Torr. The reaction gases such assilicon tetrafluoride (SiF₄), silane (SiH₄), silicon tetrafluoride andsilane, silane and germane (GeH₄), or silicon tetrafluoride and germaneare fed into the inlet chamber 24 through separate inlet conduits 46 andchamber 12 is brought up to the desired operating pressure of, forexample, 0.1 Torr. To the foregoing reaction gases, hydrogen (H₂) canalso be added. Other reaction gases which can be utilized are germane,germanium tetrafluoride (GeF₄), germanium tetrafluoride and silicontetrafluoride. Hydrogen (H₂) can also be added to these gases.

The microwave energy from the antenna 19 is directed by the reflectivehousing 21 into the vessel 12 to form a plasma in the area of thesubstrate. As a result, an amorphous semiconductor alloy film 22 isdeposited onto the substrate 14. The heater 16 maintains the substrateat a temperature between about 20° Centigrade and 400° Centigrade. Theoutput power of the microwave energy generator 18 is adjusted betweenabout 20 and 115 watts depending on the volume of the gas contained inthe plasma and the composition of the gas. These power outputspreferably correlate to about 0.1 to 1 watt per cubic centimeter inpower density. The flow rate of the reaction gases can be between 1 to10 SCCM. With the foregoing system parameters, deposition rates of 25 Åto 250 Å per second can be obtained. Even at these high depositionrates, the deposited amorphous semiconductor films exhibit high qualityphotoresponsive characteristics suitable for photovoltaic and otherapplications.

For making photovoltaic devices by the process of the invention, dopantscan be introduced into the vessel 12 for making the deposited filmeither p-type or n-type. For example, to make a p-type film, diboranegas (B₂ H₆) can be introduced through one of the inlets 46. For makingn-type films, phosphine gas (PH₃) can be introduced into one of theinlets 46. If it is desired to increase the band gap of a material,increasing elements such as carbon or nitrogen can be incorporated intothe films by introducing methane (CH₄) or ammonia (NH₃) into one of theinlets 46 during deposition. The reaction gases can be sequentiallyintroduced to result in the formation of any desired deviceconfiguration.

To assist in the maintenance of the plasma, a plasma sustaining gas canbe introduced into one of the inlets 46. Argon gas can be utilized forthis purpose. The deposition of amorphous semiconductor alloy films bythe foregoing inventive process has demonstrated many advantages overprior deposition processes. Firstly, the microwave energy provides ahigher density of free radicals than previously possible. This resultsin higher deposition rates, nearly total utilization of the feed stockreaction gases and enhanced reactivity of plasma species leading toincorporation in the growing film of elements which previously could notbe so incorporated. This results in new materials having uniquecompositional and structural properties. Secondly, the plasma formed hasa lower electron temperature. This results in substantially lower ionformation. Ion formation is believed to be deleterious to high qualityfilm deposition due to excessive bombardment of the film particularly bycharged particles such as electron by the ions. Such ion bombardmentdamages the material as it is deposited. In summary, the inventionprovides a process for making amorphous semiconductor films and deviceswhich have higher deposition rates, improved properties and whichaffords wide variations in the material compositions

Various applications of the improved amorphous alloys produced by theunique processes of the invention are illustrated in FIGS. 2 through 7.FIG. 2 shows a Schottky barrier solar cell 142 in fragmentarycross-section. The solar cell 142 includes a substrate or electrode 144of a material having good electrical conductivity properties, and theability of making an ohmic contact with an amorphous alloy 146compensated or altered to provide a low density of localized states inthe energy gap. The substrate 144 may comprise a low work functionmetal, such as aluminum, tantalum, stainless steel or other materialmatching with the amorphous alloy 146 deposited thereon which preferablyincludes silicon, compensated or altered in the manner of the alloyspreviously described so that it has a low density of localized states inthe energy gap of preferably no more than 10¹⁶ per cubic centimeter pereV. It is most preferred that the alloy have a region 148 next to theelectrode 144, which region forms an n-plus conductivity, heavily doped,low resistance interface between the electrode and an undoped relativelyhigh dark resistance region 150 which is an intrinsic, but lown-conductivity region.

The upper surface of the amorphous alloy 146 as viewed in FIG. 2, joinsa metallic region 152, an interface between this metallic region and theamorphous alloy 146 forming a Schottky barrier 154. The metallic region152 is transparent or semi-transparent to solar radiation, has goodelectrical conductivity and is of a high work function (for example, 4.5eV or greater, produced, for example, by gold, platinum, palladium,etc.) relative to that of the amorphous alloy 146. The metallic region152 may be a single layer of a metal or it may be a multi-layer. Theamorphous alloy 146 may have a thickness of about 0.5 to 1 micron andthe metallic region 152 may have a thickness of about 100 Å in order tobe semi-transparent to solar radiation.

On the surface of the metallic region 152 is deposited a grid electrode156 made of a metal having good electrical conductivity. The grid maycomprise orthogonally related lines on conductive material occupyingonly a minor portion of the area of the metallic region, the rest ofwhich is to be exposed to solar energy. For example, the grid 156 mayoccupy only about from 5 to 10% of the entire area of the metallicregion 152. The grid electrode 156 uniformly collects current from themetallic region 152 to assure a good low series resistance for thedevice.

An anti-reflection layer 158 may be applied over the grid electrode 156and the areas of the metallic region 152 between the grid electrodeareas. The anti-reflection layer 158 has a solar radiation incidentsurface 160 upon which impinges the solar radiation. For example, theanti-reflection layer 158 may have a thickness on the order of magnitudeof the wavelength of the maximum energy point of the solar radiationspectrum, divided by four times the index of refraction of theanti-reflection layer 158. If the metallic region 152 is platinum of 100Å in thickness, a suitable anti-reflection layer 158 would be zirconiumoxide of about 500 Å in thickness with an index of refraction of 2.1.

The Schottky barrier 154 formed at the interface between the regions 150and 152 enables the photons from the solar radiation to produce currentcarriers in the alloy 146, which are collected as current by the gridelectrode 156. An oxide layer (not shown) can be added between thelayers 150 and 152 to produce an MIS (metal insulator semiconductor)solar cell.

In addition to the Schottky barrier or MIS solar cell shown in FIG. 2,there are solar cell constructions which utilize p-n junctions in thebody of the amorphous alloy forming a part thereof formed in accordancewith successive deposition, compensating or altering and doping stepslike that previously described. These other forms of solar cells aregenerically illustrated in FIG. 3 as well as in FIGS. 6 and 7.

These constructions 162 generally include a transparent electrode 164through which the solar radiation energy penetrates into the body of thesolar cell involved. Between this transparent electrode and an oppositeelectrode 166 is a deposited amorphous alloy 168, preferably includingsilicon, initially compensated in the manner previously described. Inthis amorphous alloy 168 are at least two adjacent regions 170 and 172where the amorphous alloy has respectively oppositely doped regions,region 170 being shown as a n-conductivity region and region 172 beingshown as a p-conductivity region. The doping of the regions 170 and 172is only sufficient to move the Fermi levels to the valence andconduction bands involved so that the dark conductivity remains at a lowvalue achieved by the band adjusting and compensating or altering methodof the invention. The alloy 168 has high conductivity, highly dopedohmic contact interface regions 174 and 176 of the same conductivitytype as the adjacent region of the alloy 168. The alloy regions 174 and176 contact electrodes 164 and 166, respectively.

Referring now to FIG. 4, there is illustrated another application of anamorphous alloy utilized in a photodetector device 178 whose resistancevaries with the amount of light impinging thereon. An amorphous alloy180 thereof is deposited in accordance with the invention, has no p-njunctions as in the embodiment shown in FIG. 3 and is located between atransparent electrode 182 and a substrate electrode 184. In aphoto-detector device it is desirable to have a minimum darkconductivity and so the amorphous alloy 180 has an undoped, butcompensated or altered region 186 and heavily doped regions 188 and 190of the same conductivity type forming a low resistance ohmic contactwith the electrodes 182 and 184, which may form a substrate for thealloy 180.

Referring to FIG. 5 an electrostatic image producing device 192 (like axerography drum) is illustrated. The device 192 has a low darkconductivity, selective wavelength threshold, undoped or slightlyp-doped amorphous oxygen stabilized alloy 194 deposited on a suitablesubstrate 196 such as a drum.

As used herein, the terms compensating agents or materials and alteringagents, elements or materials mean materials which are incorporated inthe amorphous alloy for altering or changing the structure thereof, suchas, activated fluorine (and hydrogen) incorporated in the amorphousalloy containing silicon to form an amorphous silicon/fluorine/hydrogencomposition alloy, having a low density of localized states in theenergy gap. The activated fluorine (and hydrogen) is bonded to thesilicon in the alloy and reduces the density of localized states thereinand due to the small size of the fluorine and hydrogen atoms they areboth readily introduced into the amorphous alloy without substantialdislocation of the silicon atoms and their relationships in theamorphous alloy.

Referring now to FIG. 6, a p-i-n solar cell 198 is illustrated having asubstrate 200 which may be glass or a flexible web formed from stainlesssteel or aluminum. The substrate 200 is of a width and length as desiredand preferably at least 3 mils thick. The substrate has an insulatinglayer 202 deposited thereon by a conventional process such as chemicaldeposition, vapor deposition or anodizing in the case of an aluminumsubstrate. The layer 202 for instance, about 5 microns thick can be madeof a metal oxide. For an aluminum substrate, it preferably is aluminumoxide (Al₂ O₃) and for a stainless steel substrate it may be silicondioxide (SiO₂) or other suitable glass.

An electrode 204 is deposited in one or more layers upon the layer 202to form a base electrode for the cell 198. The electrode 204 layer orlayers is deposited by vapor deposition, which is a relatively fastdeposition process. The electrode layers preferably are reflective metalelectrodes of molybdenum, aluminum, chrome or stainless steel for asolar cell or a photovoltaic device. The reflective electrode ispreferably since, in a solar cell, non-absorbed light which passesthrough the semiconductor alloy is reflected from the electrode layers204 where it again passes through the semiconductor alloy which thenabsorbs more of the light energy to increase the device efficiency.

The substrate 200 is then placed in the deposition environment. Thespecific examples shown in FIGS. 6 and 7 are only illustrative of somep-i-n function devices which can be manufactured utilizing the improvedprocess of the invention. For example, tandem cells can also be made bythe process of the present invention. Each of the devices illustrated inFIGS. 6 and 7, has an alloy body having an overall thickness of betweenabout 3,000 and 30,000 angstroms. This thickness ensures that there areno pin holes or other physical defects in the structure and that thereis maximum light absorption efficiency. A thicker material may absorbmore light, but at some thickness will not generate more current sincethe greater thickness allows more recombination of the light generatedelectron-hole pairs. (It should be understood that the thicknesses ofthe various layers shown in FIGS. 2 through 7 are not drawn to scale.)

Referring first to forming the n-i-p device 198, the device is formed byfirst depositing a heavily doped n⁺ alloy layer 206 on the electrode204. Once the n⁺ layer 206 is deposited, an intrinsic (i) alloy layer208 is deposited thereon. The intrinsic layer 208 is followed by ahighly doped conductive p⁺ alloy layer 210 deposited as the finalsemiconductor layer. The amorphous alloy layers 206, 208 and 210 formthe active layers of the n-i-p device 198.

While each of the devices illustrated in FIGS. 6 and 7 may have otherutilities, they will be now described as photovoltaic devices. Utilizedas a photovoltaic device, the selected outer, p⁺ layer 210 is a lowlight absorption, high conductivity alloy layer. The intrinsic alloylayer 208 preferably has an adjusted wavelength threshold for a solarphotoresponse, high light absorption, low dark conductivity and highphotoconductivity. The bottom alloy layer 204 is a low light absorption,high conductivity n⁺ layer. The overall device thickness between theinner surface of the electrode layer 206 and the top surface of the p⁺layer 210 is, as stated previously, on the order of at least about 3,000angstroms. The thickness of the n⁺ doped layer 206 is preferably in therange of about 50 to 500 angstroms. The thickness of the amorphousintrinsic alloy 208 is preferably between about 3,000 angstroms to30,000 angstroms. The thickness of the top p⁺ contact layer 210 also ispreferably between about 50 to 500 angstroms. Due to the shorterdiffusion length of the holes, the p⁺ layer generally will be as thin aspossible on the order of 50 to 150 angstroms. Further, the outer layer(here p⁺ layer 210) whether n⁺ or, p⁺ will be kept as thin as possibleto avoid absorption of light in the contact layer.

A second type of p-i-n junction device 212 is illustrated in FIG. 7. Inthis device a first p⁺ layer 214 is deposited on the electrode layer204' followed by an intrinsic amorphous alloy layer 216, an n amorphousalloy layer 218 and an outer n⁺ amorphous alloy layer 220. Further,although the intrinsic alloy layer 208 or 216 (in FIGS. 6 and 7) is anamorphous alloy, the other layers are not so restricted and could, forinstance, be polycrystalline, such as layer 214. (The inverse of theFIGS. 6 and 7 structure not illustrated, also can be utilized.)

Following the deposition of the various semiconductor alloy layers inthe desired order for the devices 198 and 212, a further deposition stepis performed, preferably in a separate deposition environment.Desirably, a vapor deposition environment is utilized since it is a fastdeposition process. In this step, a TCO layer 222 (transparentconductive oxide) is added which, for example, may be indium tin oxide(ITO), cadmium stannate (Cd₂ SnO₄), or doped tin oxide (SnO₂). The TCOlayer will be added following the post compensation of fluorine (andhydrogen) if the films were not deposited with one or more of thedesired compensating or altering elements therein. Also, the othercompensating or altering elements, above described, can be added by postcompensation.

An electrode grid 224 can be added to either of the device 198 or 212 ifdesired. For a device having a sufficiently small area, the TCO layer222 is generally sufficiently conductive such that the grid 224 is notnecessary for good device efficiency. If the device is of a sufficientlylarge area or if the conductivity of the TCO layer 222 is insufficient,the grid 224 can be placed on the layer 222 to shorten the carrier pathand increase the conduction efficiency of the devices.

Lastly, a transparent encapsulant 225 is deposited over the grid 224.This encapsulant can comprise, for example, silicon nitride (Si₃ N₄) orsilicon dioxide (SiO₂) formed from the microwave deposition of silaneand nitrogen or ammonia or silane and oxygen respectively. The layer 225or transparent material can have a thickness of about one to fiftymicrons.

If the layer 225 comprises silicon nitride, the reaction gases can besilane (SiH₄) and a mixture of three percent hydrogen and ninty-sevenpercent nitrogen. The deposition temperature can be room temperature andthe deposition pressure can be between 0.7 and 1.5 Torr. The reactiongas flow range can be about 10 SCCM and the power between 80 and 100watts.

Each of the device semiconductor alloy layers can be deposited upon thesubstrate by the apparatus illustrated in FIG. 1. The vessel 12initially is evacuated to approximately 0.1 Torr to purge or eliminateimpurities in the atmosphere from the deposition system. The alloymaterial preferably is then fed into the deposition chamber in acompound gaseous form, most advantageously as a semiconductor containingcompounds for intrinsic materials. The reactive gas can contain band gapadjusting elements such as germanium to form an intrinsic amorphoussemiconductor alloy having a narrowed band gap. The microwave generatoris energized and the plasma is obtained from the gas mixture.

The semiconductor material is deposited from the plasma onto thesubstrate which can be heated to the desired deposition temperature foreach alloy layer. For example, the substrate temperature can be 275° C.for amorphous silicon and germanium alloys and 200° C. for amorphousgermanium alloys deposited from GeF₄ or GeH₄. The doped layers of thedevices are deposited at various temperatures of for example 250°0 C. to300° C. depending upon the form of the material used. The upperlimitation on the substrate temperature in part is due to the type ofmetal substrate utilized. For an initially hydrogen compensatedamorphous alloy to be produced, such as to form the intrinsic layer inn-i-p or p-i-n devices, the substrate temperature should be less thanabout 400° C. and preferably about 275° C.

The doping concentrations are varied to produce the desired p, p⁺, n orn⁺ type conductivity as the alloy layers are deposited for each device.For n or p doped layers, the material is doped with 5 to 100 ppm ofdopant material as it is deposited. For n⁺ or p⁺ doped layers thematerial is doped with 100 ppm to over 1 percent of dopant material asit is deposited.

Referring now to FIG. 8, there is illustrated an alternative gas feedsystem for the apparatus of FIG. 1. The gas feed system includes a gasdistribution manifold 230 within the enclosed chamber 24. The manifold230 has an extension 232 which extends through the chamber end cap (notshown) for receiving the various gas mixtures to be utilized in themicrowave plasma. The gas mixtures can be any of gas mixtures previouslyidentified. As can be seen in FIG. 8, the manifold loops around thesubstrate 14 and includes a plurality of outlets along substantiallyparallel portions 234 and 236. This allows the reaction gases indicatedby arrows 238 to be evenly distributed over the substrates to result ina more uniform plasma. As a result, the amorphous semiconductor alloyfilm deposited onto the substrate 14 will have uniform electrical andoptical properties across the substrate 14. Such an arangement isadvantageous when using gases such as, for example, silicontetrafluoride and germane, or silicon tetrafluoride and germaniumtetrafluoride wherein the silicon compounds and germanium compounds havedifferent disassociation energies and consequently would otherwiseresult in a deposited film which exhibits a compositional non-uniformityin the direction of feed gas flow across the substrate.

Referring now to FIG. 9, there is illustrated a system of feeding intothe plasma atomic fluorine and/or hydrogen which have been separatelygenerated. The system includes a pair of conduits 240 and 242 whichextend into the chamber 24 on opposite sides of the substrate 14. Theconduits 240 and 242 are substantially equally spaced from the substrateand include outlets in the vicinity of the substrate for evenlydistributing atomic fluorine and/or hydrogen (indicated by arrows 244and 246) into the plasma over the substrate 14. The atomic fluorineand/or hydrogen can then react with the semiconductor free radicalswithin the plasma disassociated from the semiconductor containing gases248 fed into the chamber 24 through inlets 46 (not shown). The atomicfluorine and/or hydrogen and the semiconductor free radicals react toform a film on the substrate. As a result, the system of FIG. 9 providesseparate control over the free radicals within the plasma to enableselective incorporation of desired species into the plasma from whichthe film is deposited.

Other free radicals can of course be introduced by adding additionalconduits. For example, free radicals of boron can be introduced toprovide substitutional doping within the deposited film to form animproved p-type alloy. Such an alloy is particularly useful in makingphotovoltaic devices.

Referring now to FIG. 10, there is illustrated another microwavedeposition in accordance with a further embodiment of the invention. Inthis system a free radical generator 250 including "Woods Horn" 254known in the art is used to feed selected free radicals 252 into thechamber 24. Of course, additional generators 250 can be provided. Amicrowave source as in FIG. 1 can be provided including a microwavegenerator 18, an antenna 19, and a reflective housing 21. The generator18 can provide the free radical generator 250 with microwave energy, orthe free radical generator 250 can include its own source of microwaveenergy.

The free radicals 252 react with the reactive species formed within theplasma from the reaction gases 256 to form a film on the substrate 14.Hence, as in the previous embodiment, selected free radicals can beintroduced into the plasma at will to form new and improved amorphoussemiconductor alloys.

The atomic fluorine and/or hydrogen within the plasma provides amorphoussemiconductor alloys having improved structural and chemical properties.Infra-red spectroscopy shows a significant silicon-fluorine peak in thealloys indicating that the fluorine is bonding to the silicon in apreferred manner providing material stability and reduced density ofstates. This is of particular importance in the fabrication ofphotovoltaic devices.

As previously mentioned, the alloy layers other than the intrinsic alloylayer can be other than amorphous layers, such as polycrystallinelayers. (By the term "amorphous" is meant an alloy or material which haslong range disorder, although it may have short or intermediate order oreven contain at times some crystalline inclusions.)

Modifications and variations of the present invention are possible inlight of the above teachings. It is therefore, to be understood thatwithin the scope of the appended claims the invention may be practicedotherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A process for depositing thin, non-single crystalfilms onto a substrate, comprising:providing a source of microwaveenergy; providing an enclosed reaction vessel; providing a substrate insaid reaction vessel; coupling said microwave energy into saidsubstantially enclosed reaction vessel containing said substrate;introducing into said vessel at least one reaction gas to form a glowdischarge plasma within said vessel and to form reaction gas speciesfrom said reaction gas; and evacuating said reaction vessel to adeposition pressure of 0.1 torr or less so as to provide for thedeposition of a film from said reaction gas species onto said substrateat high deposition rates with high reaction gas conversion efficiencies.2. A process as defined in claim 1 wherein at least a semiconductorcontaining reaction gas is introduced into the vessel.
 3. A process asdefined in claim 1 wherein hydrogen is also introduced into the vessel.4. A process as defined in claim 1 further comprising the step ofintroducing a plasma sustaining gas into the vessel.
 5. A process asdefined in claim 4 wherein said plasma sustaining gas is argon.
 6. Aprocess as defined in claim 4 wherein said plasma sustaining gas ishydrogen.
 7. A process as defined in claim 2 wherein deposited film is asemiconductor having a band gap capable of promoting an electron fromthe valence to the conduction band thereof.
 8. A process as defined inclaim 7 wherein a wide band gap semiconductor containing gas isintroduced into the vessel.
 9. A process as defined in claim 8 whereinsaid wide band gap semiconductor reaction gas includes at least methanegas (CH₄).
 10. A process as defined in claim 1 further including thestep of maintaining the temperature of said substrate between about 20°Centigrade and 400° Centigrade.
 11. A process as defined in claim 9further including the step of adjusting the power output of saidmicrowave energy source to provide power densities between about 0.1 to1 watt per cubic centimeter.
 12. A process as defined in claim 1 whereinthe frequency of said microwave energy is 2.45 Gigahertz.
 13. A processfor depositing a thin film, non-single crystal layer of wide band gap,substantially transparent, hard, chemically inert materialcomprising:providing a source of microwave energy; providing an enclosedreaction vessel; providing a substrate in said reaction vessel; couplingsaid microwave energy into said substantially enclosed reaction vessel;introducing into said vessel at least one reaction gas to form a glowdischarge plasma within sasid vessel and to form reaction gas speciesfrom said reaction gas; and evacuating said reaction vessel to adeposition pressure 0.1 torr or less so as to provide for the depositionof a transparent material from said reaction gases onto said substrate.14. A process as defined in claim 13 wherein said reaction gas includesmethane.
 15. A process as defined in claim 14 wherein said reaction gasfurther includes hydrogen.
 16. A process as defined in claim 13including the step of continuing said deposition at least until saidtransparent material covers the subjacent surface.
 17. A process asdefined in claim 1 including the further step of selecting the reactiongases introduced into said vessel so as to deposit an insulating siliconalloy film, which film incorporates into the matrix thereof a materialselected from the group consisting essentially of oxygen, nitrogen,carbon, and combinations thereof.